1. Field of the Invention
The present invention generally relates to a developing apparatus for use in an electrophotographic apparatus or the like and, more particularly, to a developing apparatus utilizing the action of an electric field curtain.
2. Description of the Prior Art
Conventionally, for a developing apparatus to be used in an electrophotographic apparatus widely applied to a copying machine, facsimile, printer etc. not only two-component system by the cascade method or magnetic brush method etc., but one-component system has been well employed. The one-component system has been disclosed in, for example, U.S. Pat. No. 2,725,304 by the power cloud method, Japanese Patent Publication Nos. 41-9475 by the touch-down method, 45-2877 by the impression method or 54-3624 by the jumping method.
Meanwhile, because of the recent trend in the electrophotography requiring colored images, such kind of a developing apparatus has attracted the notice of people that can develop image while overlapping developing materials of various colors onto an image holding member such as a photosensitive member, without contact between the image holding member and a developing material support member.
In the developing apparatus of the type referred to above which can develop image without contact of the image holding member with the developing material support member, utilization of the aforementioned powder cloud method or jumping method has been well practiced.
According to the powder cloud method, however, since the developing apparatus is so arranged that charged toners are jetted out by a flow of air into a cloud of toners, and the image holding member formed with an electrostatic latend image thereon passes through the toner cloud, thereby realizing the supply of charged toners to the developing material support member through the image holding member, the apparatus tends to be complicated in structure and bulky in size, having difficulties in control of the toner cloud floating in the air.
On the other hand, in the developing apparatus employing the jumping method, toners move reciprocally to and fro between the developing material support member and the image holding member, and therefore it is difficult to obtain an image of high resolution. Moreover, toners of various colors are prone to be mixed during the development of an image in an overlapping manner.
Taking note of the above drawbacks, recently, a developing apparatus utilizing the action of an electric field curtain has been developed which supplies the developing material without contact thereof with the image holding member, such as disclosed in Japanese Patent Laid-open Publication Nos. 47-47811, 58-202217, 59-176755, 59-181371, 59-189367, 63-13068 and so on.
The developing apparatus utilizing the action of the electric field curtain is fundamentally in such arrangement, as shown in FIG. 1, that an alternating voltage is applied between electrodes 1 by an alternating voltage source 2, thereby causing a non-uniform alternating field having an electric line of force as indicated by a broken line to act between the electrodes 1, with allowing toners 3 to float to a developing area confronting the image holding member, and there in the developing area, the toners are supplied to a position of an electrostatic latent image on the image holding member by the action of the electric field curtain.
The above electric field curtain will be more clearly described hereinbelow with reference to FIG. 1.
As mentioned above, as charged toners 3 come close to the electric field area where the electric line of force is formed by the application of the alternating voltage from the alternating voltage source 2 to the electrodes 1, the toners 3 receive an alternating electric force in a direction along the electric line of force, and accordingly the toners 3 vibrate approximately along the curved electric line of force.
In addition to an external force Fe such as gravity etc., the toners 3 receive, every half cycle of the vibration, a centrifugal force Fc outwardly perpendicular to a tangent line of the electric line of force, and further receive an average gradient force Fg along the electric line of force. The centrifugal force Fc and average gradient force Fg act following the spatial change respectively in a direction of the electric line of force and in a density of the electric line of force, both of which result from the non-uniform and alternating electric field.
An electrodynamic force Fr (=Fc+Fg) acting on the charged toners 3 charged by the standing wave electric field curtain is represented as follows: ##EQU1## wherein m[Kg] denotes a mass of a toner particle, a[m] denoting a radius of the toner particle, q[c] a charge amount of the toner particle, .omega.[s.sup.-1 ] an angular frequency of a power source, .eta.[N.s/m.sup.2 ] an air viscous resistance, E[V/m] a field intensity, V[V] an impressing voltage, b[m] a minimum distance between surfaces of neighboring electrodes, Emax[V/m] a maximum field intensity on the surface of an electrode, H=(v/b)/Emax an utilization rate of electric field, f=E(V/b) a standardized electric field, (grad)b a grad in a coordinates system (r/b, z/b) standardized by b, respectively.
More specifically, the force by the electric field curtain depends on conditions at the side of the toner particle constituted of the mass m of the toner particle, the radius a of the toner particle and the charge amount q of the toner particle, and also external conditions, that is, the field intensity E and the source frequency f(=w/2.pi.). Therefore, it becomes possible to float the toner particles in the air by adjusting the above conditions properly
Accordingly, in the developing apparatus utilizing the action of the electric field curtain, toners can be floated by the action of the electric field curtain as described above, and brought into soft contact with the image holding member to be fed onto the position of the electrostatic latent image. Therefore, in comparison with the developing apparatus utilizing the powder cloud method or jumping method, overlapping development with many colors becomes well achieved, although it is necessary to charge toners sufficiently in transportation thereof.
In view of the above, some improving idea has been proposed as disclosed in Japanese Patent Laid-open Publication No. 58-202217 and 63-13066 etc. in which a group of electrode acting the effect of the electric field curtain is partially exposed to carry out corona discharge thereby, so that the toners are forcibly charged.
It is required, however, to apply a remarkably high voltage to the electrodes for transporting the toners charged by the corona discharge by the action of the electric field curtain, which leads to deterioration of insulation of a dielectric member in a short period of time, with reducing the service life of the apparatus itself. Furthermore, it causes a number of problems such as generation of ozone and noises.
Moreover, in the case where toners which are not charged to a proper charge amount are floated for transportation by the action of the electric field curtain, the insufficiently-charged toners may be scattered or fogging may be formed in a resultant image.